This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. New evidence suggests that autistic disorder (AD) may be associated with abnormalities in folate metabolism, which is a process that affects genetic expression by facilitating the formation of methyl donors for DNA methylation. Limited data show that some children with AD show behavioral improvements with folic acid (FA) therapy, while others show a worsening effect. If behavioral worsening is linked with abnormalities in folate metabolism, then nutritional modifications could normalize these processes and result in clinical improvements. To address this premise, we propose a randomized, placebo-controlled crossover pilot study with two phases. The first phase will focus on the behavioral and biochemical responses of children with AD to high-dose folic acid supplementation. Because FA is an inactive folate that requires biochemical conversion to become active, and select genotypes impede this conversion, our general hypothesis is that FA will yield behavioral improvements in some children but exacerbate problem behaviors in others. During the second phase, children who had a worsened behavioral response to FA during phase 1 will participate in an open-label trial of high-dose Metafolin\'ae supplementation. The focus here would similarly be on the behavioral and biochemical outcomes of participating children following treatment with the study supplement. Because Metafolin\'ae is an active folate metabolite that should not be affected by genotypes in the folate pathway, our general hypothesis for phase 2 is that Metafolin\'ae would yield behavioral improvements without the risk for behavioral worsening. Results from this project may provide support for continued study of the potential relationship between folate metabolism and problem behaviors among children with AD, potentially justifying the need to examine effects of folate supplementation among a larger sample of affected children. HYPOTHESIS 1. Children who reportedly experienced a negative behavioral response to prior folate therapy will show a worsened behavioral response to FA treatment. 2. Differences in children's behavioral outcomes will depend on genotypes for dihydrofolate reductase (\i DHFR\i0 ), cytoplasmic and mitochondrial serine hydroxymethyltransferase (\i SHMT1\i0 and \i SHMT2\i0 , respectively), methylene tetrahydrofolate reductase (\i MTHFR\i0 ), reduced folate carrier (\i RFC-1\i0 ), methionine synthase reductase (\i MTRR\i0 ), 5-methyltetrahydrofolate-homocysteine S-methyltransferase (\i MTR\i0 alias methionine synthase), and/or transcobalamin II (\i TCII\i0 ). Children who have a worsened behavioral response to FA will not have a worsened behavioral response to Metafolin\'ae. Children will exhibit significant changes in levels of folate-related metabolites while on FA or Metafolin\'ae. The folate-related metabolites of interest include FA as pteroyglutamic acid, 5-methyltetrahydrofolate (5-MTHF), 5-10 methyltetrahydrofolate (5-10 MTHF), methionine, homocysteine, S-adenosyl methionine (SAM), S-adenosyl homocysteine (SAH), and free folic acid (only in phase 1). Levels of folate-related metabolites will correlate with behavioral response. Abnormal levels of unmetabolized FA and folate-related metabolites will be related to more maladaptive behaviors while normal levels will be related to less maladaptive behaviors. DNA methylation will be increased among children while on FA or Metafolin\'ae. To examine potential changes in the maladaptive behaviors of children with AD, as measured with the Aberrant Behavior Checklist (ABC) and the \ldblquote Arousal\rdblquote and \ldblquote Aggressiveness\rdblquote domains of the Pervasive Developmental Disorder Behavior Inventory (PDDBI), following supplementation with high-dose FA (phase 1) and high-dose Metafolin\'ae (phase To examine potential changes in the levels of folate-related metabolites in plasma among children with AD following supplementation with high-dose FA (phase 1) and high-dose Metafolin\'ae (phase 2). To examine potential changes in DNA methylation among children with AD following supplementation with high-dose FA (phase 1) and high-dose Metafolin\'ae (phase 2).